1. Field of the Invention
The present invention relates to a rush current limiting device to be provided for a power circuit.
2. Description of the Related Art
FIG. 8 is a circuit diagram showing structure of a rectifying smoothing circuit for an AC power source to be provided for, for example, a switching power unit or the like.
In this circuit, an AC power source inputted from a convenience outlet 10 is rectified by a rectifier diode bridge 16 through a switch 12 and a fuse 14, and is smoothed by a smoothing condenser 18 to be supplied to a converter circuit (not shown) or the like in the latter stage.
This circuit is provided with a resistance 20 for limiting rush current when the switch 12 is thrown in the preceding stage of the smoothing condenser 18. This prevents a circuit element (rectifying device, fuse or the like) from being damaged because excessive current flows when the switch 12 is thrown.
In a case, however, where there is provided such a resistance 20, in the case of a large-output power unit having a small difference between current value to be limited and current value to be inputted to the power source, or the like, there is the problem that the amount of heat generated by the resistance 20 becomes great because electric power consumed by a limiting resistance 20 during operation becomes great.
There is already known a circuit in which, as shown in FIG. 9, the current when the switch 12 is thrown is limited using a thermistor 22 in place of the resistance 20, and thereafter the current can be increased by decreasing the resistance by self-heat generation of the thermistor 22.
There is also known a circuit in which , as shown in FIG. 10, a relay 24 is provided in parallel with the resistance 20, this relay is controlled by a control circuit 26, and after the switch 12 is thrown, the resistance 20 is short-circuited at fixed timing to thereby restrain the power consumption by the resistance 20.
There is further known a circuit in which, as shown in FIG. 11, a triode AC switch 28 is provided in parallel with the resistance 20. This triode AC switch is controlled by a control circuit 30, and after the switch 12 is thrown, the resistance 20 is short-circuited at fixed timing to thereby restrain the power consumption by the resistance 20.
In a case, however, where the thermistor 22 is used as shown in FIG. 9, it becomes impossible to limit the rush current while the resistance value remains low for a condition that the switch 12 is thrown again with the thermistor 22 in a high-temperature state.
In a case where the relay 24 is used as shown in FIG. 10, while a fixed amount or more of current is flowing through the resistance at a timing at which the relay 24 operates, the voltage difference is to short-circuit the relay 24, and large current flows at that point of time. In order to prevent this flow, it is necessary to strictly control the operation timing of the relay 24, but it is, in many instances, impossible to control when using a mechanical relay 24.
Also, in a case where the triode AC switch 28 is provided as shown in FIG. 11, it is comparatively easy to control the timing, but there is the problem that when the input current becomes great, the power consumption at the triode AC switch 28 becomes great owing to the existence of on-voltage (about 1V) of the triode AC switch 28, thus causing it to generate heat.